Positional cloning - the identification of obesity genes by co-incidence of mapped obesity traits andgenes to a specific chromosomal region, provides a powerful method to discover novel obesity genes and to make novel insights into the mechanisms causing obesity. Mice have been used for the first identification of more human obesity genes than any other model. The goal of this proposal is to use positional cloning methods to identify obesity genes in mouse congenic strains. Congenic mouse strains are identical to a background strain except for a chromosomal region from a donor strain. Phenotype differences between congenic and background are due to alleles of the donor strain with functional effects different from the background strain. The two laboratories contributing to this proposal have identified several mouse congenic strains with statistically significant phenotypes for obesity on mouse chromosome 2 in a region homologous to human chromosome 20, where several obesity quantitative trait loci have been mapped. Our general hypothesis is that one or more genes or transcripts in the congenic donor regions influences obesity. Our specific working hypothesis is that genes/transcripts underlying obesity can be identified for most congenics. We propose seven Specific Aims. We will: determine diet, sex and age effects on obesity in the founding congenics (Aim 1), identify minimal chromosomal loci containing obesity genes by breeding congenics with ever smaller donor regions that retain obesity phenotypes (Aim 2), find differentially expressed donor region genes with whole genome microarrays using RNAfrom eight obesity tissues (Aim 3), sequence selected donor region genes (Aim 4), determine cis or trans control of mRNA levels for differentially expressed genes (Aim 5), produce transgenic mice overexpressing ten candidate genes prioritized by the results of Aims3-5 (Aim 6) and identify microRNAs or other transcribed non-coding genes that may influence obesity (Aim 7). The long term outcome is that we will identify at least one novel and strong and plausible obesity gene.